fun <T> Array<out Array<out T>>.flatten(): List<T>
Returns a single list of all elements from all arrays in the given array.
fun <T> Iterable<Iterable<T>>.flatten(): List<T>
Returns a single list of all elements from all collections in the given collection.
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Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/flatten.html